The present invention relates to a color image forming apparatus for obtaining images electrophotographicly that a toner image is formed on a belt-like image carrier and is transferred onto a transfer material.
There have been many prior disclosures about electrophotographicly reproducing color images. In Japanese Patent Laid-Open 61-100770, for example, a photoreceptor drum has a latent image formed thereon and is developed corresponding to the number of separated colors of an original image. The image is transferred to a transfer drum every time it is developed to form a multi-color image on the transfer drum. After this, it is transferred onto a recording paper to obtain a color copy. However, this apparatus has a disadvantage that it is needed to have a transfer drum is necessary, which is large enough that a whole page of image can be transferred on the surface thereof. The apparatus is unavoidably large and complicated in construction.
As another example, in the apparatus disclosed in Japanese Patent Laid-Open 61-149972, a photoreceptor drum has a latent image formed thereon and is developed corresponding to the number of separated colors of an original image. The image is transferred to a transfer material every time a color is developed to reproduce a multi-color copy. However, the apparatus described above has the disadvantage that it is hard to accurately register a plurality of color images.
As another example of conventional apparatus, color toner images are registered on a photoreceptor drum by repeatedly forming latent images corresponding to the number of separated colors of an original image on the photoreceptor drum and developing them with color toners. The registered image is then transferred to obtain a color image. This basic process for forming a multi-color image is disclosed in, for example, Japanese Patent Laid-Open 60-75850, 60-76766, 60-95456, 60-95458, and 60-158475.
In multi-color image forming apparatus in which images are registered to obtain a color image, there are provided around the photoreceptor drum a plurality of developing sections containing toners of different colors. The photoreceptor drum is rotated a plurality of times to develop the latent images on the photoreceptor drum.
For the image carrier, a belt-type image carrier having a photoconductive substance coated or mounted on a belt may be used in addition to the photoreceptor drum having the photoconductive substance coated or evaporated on the surface thereof as described above. The belt-like image carrier (hereinafter also referred to as the photoreceptor belt) is desirable for a compact color image forming apparatus to more effectively use space, because its shape is determined by being stretched between revolving rollers including a drive roller. Also, it is desirable in that the photoreceptor belt can be conveyed with a small curvature so that the transfer material can be separated by making use of the curvature of a revolving roller of small diameter. This ensures the transfer material is surely separated.
In color image forming apparatus using the photoreceptor belt, there are provided around the photoreceptor belt; charging means, image exposing means, and developing means comprising a plurality of developing sections. These image forming means face the revolving photoreceptor belt at predetermined distances.
As the image forming means face the photoreceptor belt with certain gaps, backup rollers have been used. However, this method has the disadvantage that it is necessary to arrange a plurality of backup rollers corresponding to the number of the image forming means. If the number of rollers is too much, it is hard to keep parallelism. Further, there is another apparatus disclosed in Japanese Patent Laid-Open 57-34576 in which the image forming means face the photoreceptor belt on a guide member. However, it is hard to keep precise gaps in such an apparatus.